1. Field of the Invention
The invention relates to rotary end face seals used to contain a pressurized medium by sealing the annulus between a stationary housing and a rotary shaft, for example, as found in pumps in which a drive shaft penetrates the pressure boundary.
2. Brief Description of Prior Art
Rotary end face seals relating to this invention are advantageously employed over the full spectrum of parameters defining present industrial service and operating conditions. One of the more demanding applications of rotary end face seals is in the nuclear industry where, for example, primary pumps circulate reactor coolant at pressures and temperatures ranging above 2000 psig and 550.degree. F. respectively. Seal support systems attempt to maintain a normal operating seal environment temperature below about 150.degree. F., but fluctuating fluid temperatures and short term excursions above 200.degree. F. can be experienced due to support system transients or loss of some support systems; higher environment temperatures can result from loss of all seal support systems. Seal environment pressures also fluctuate in response to reactor operating conditions, and the pumps (and seals) may be stopped or started at various pressures or temperatures.
A typical rotary end face seal assembly includes a rotary assembly and a stationary assembly, with either assembly axially movable and spring mounted. Either the rotary assembly or the stationary assembly has a component with a protruding seal face in close sliding proximity with the seal face of a component of dissimilar material of the mating assembly. Each of the components with the seal faces is supported by a support ring of some kind, sometimes referred to as a back-up ring, carrier ring, holder or other.
Seal performance, in terms of effectiveness and lifespan is determined primarily by the condition of the two seal faces and the sealing gap between them. As installed, the seal faces must be finished precisely to within lightbands of the specified flatness, concavity or convexity. Maintenance of the integrity of the sealing gap and seal faces during service is the prime objective of seal design and quality control.
With an increase in pressure, all exposed seal components will deform axially and radially due to direct pressure and due to interaction forces, normal and shear, generated between contacting components as a result of the action of pressure. A change in seal assembly design will alter the magnitude of the interactive forces. The seal ring sealing faces will generally suffer distortion, resulting in change of the sealing gap profile. Too much distortion leads either to excessive leakage or to excessive seal face rubbing contact with resulting damage to the seal faces. Subsequent changes in pressure cannot restore the integrity of damaged seal faces and sealing gap and sealing performance are permanently reduced.
Although the magnitude of the pressure-induced axial forces of component interaction are proportional to pressure magnitude, radial shear forces resulting from friction between components can be non-linear and can change direction on substantial reversal of pressure change. The likelihood of occurrence of sliding between each seal ring and its support or back-up ring is heightened by the great disparity in their values of Young's modulus.
Although homogeneous, unconstrained components experiencing uniform temperature change will deform symmetrically with unaltered stress state, imposed constraints will alter the deformation and stress state. That is, axial and radial forces between contacting components may change with uniform temperature change of all components, with a consequent distortion of the seal face profiles and the sealing gap, again leading to either excessive leakage or to seal face rubbing contact with resulting damage to the seal faces. Non-equal bulk changes in temperature among constrained components will also cause altered states of contacting forces and related stress and deformation states with consequent undesirable sealing performance. Internally non-uniform component temperature changes will cause non-uniform component deformation and, if constrained, an altered component stress state. In operation, the seal assembly would see a complicated combination of the above idealized temperature changes.
Complicating the situation of pressure and thermally induced deformation and change in stress state, and exaggerating the deleterious effects on seal performance is the use of dissimilar materials for contacting components. In particular, differential radial deformation due to pressure and temperature change of relatively statically contacting components of dissimilar materials will generate radial shear forces which can vary non-linearly and change direction with temperature and pressure changes.
Carbon graphite is a typical material of choice for the protruding seal face component because of its low characteristic sliding friction and the resultant low wear rate. Other crucial properties of carbon graphite such as its Young's modulus, strength, thermal expansion coefficient and permeability are highly variable and highly discrepant relative to the materials of choice for the other components of the seal assembly Generally, to resist high operating pressures, the carbon graphite component must be of a relatively large cross-section. For large-size carbon graphite rings the effects of the non-homogeneity of its properties are magnified, contributing to non-uniform and unpredictable response, uneven and excessive wear and seal leakage, and frequently premature and unexpected seal failure with its attendant risk of hazardous consequences, loss of benefits and cost of remedial actions.
In typical seals, the difference in mechanical and material properties between the carbon graphite ring and its support ring, as well as between the mating seal face component and its support ring, results in a non-linear response to operating conditions, particularly to changes of pressure and temperature. As a result of the intimate interaction of each sealing ring and its support or back-up ring, the sealing conditions at the sealing interface suffer, leading to a highly non-linear relationship between pressure change or temperature change and leakage. This undesirable seal characteristic is known as seal face deflection hysteresis.
Many rotary end face seal assemblies also experience unstable performance characterized by erratic and excessive leakage and seal face wear as a result of thermal sensitivity inherent in the design of their components. When such seal assemblies are placed in a tandem (series) arrangement for the purpose of sharing a high pressure drop, the temperature and pressure sensitivities, or the pressure sensitivity alone, can initiate a complex, undesirable, unstable pressure oscillation between succeeding seal cavities, resulting in a further escalation of seal face deterioration and premature seal failure.
Therefore, typical rotary end face seal performance under fluctuating operating conditions is plagued by instability, unreliability and unsatisfactory seal life. This invention addresses these problems of seal performance with novel features yielding a more stable and predictable performance and longer life.